Voltage-dependent Ca.sup.2+ channels are known to exist in cardiac, skeletal and smooth muscle cells as well as excitable and secretory cells. 1,4-Dihydropyridines are potent blockers of the voltage-dependent Ca.sup.2+ channel. The 1,4-dihydropyridine receptor has been found to be highly enriched in the transverse tubular membranes of skeletal muscle. Although the dihydropyridine receptor has been purified from transverse tubular membranes of skeletal muscle, its subunit composition remains to be elucidated completely. Curtis et al have shown that it consists of three polypeptides of 160,000 dalton (Da), 50,000 Da and 32,000 Da and that under reducing conditions the apparent molecular weight 160,000 Da subunit shifted to 130,000 Da. Borsotto et al have identified three polypeptides of 142,000 Da, 33,000 Da and 32,000 Da in their preparation of the dihydropyridine receptor. Furthermore, they have shown by immunoblotting with polyclonal antibodies that the 142,000 Da and 32,000 Da subunits are produced by the reduction of a 170,000 Da polypeptide. Recently, Flockerzi et al, using a modification of the procedure of Curtis et al, have shown that the dihydropyridine receptor contains four subunits of 142,000 Da, 122,000 Da, 56,000 Da and 31,000 Da.
The 1,4-dihydropyridine receptor has been purified from rabbit skeletal muscle triads and monoclonal antibodies have been produced that are capable of specifically immunoprecipitating labeled receptor as described hereafter.